CASTLE: NORTH AMERICAN RHYNCHOBDELLID. 59 
VI. PARASITES. 
Three different endo-parasites, of which I find no notice in the litera- 
ture, in addition possibly to one already described by Bolsius (96), 
infest more or less commonly the species of Glossiphonia found in the 
vicinity of Cambridge, Mass. One of these is a small nematode, another 
a trematode, these two having been observed in the body of G. stagnalis 
only ; the third is a sporozodn found in at least four of the species 
described in this paper. 
In January, 1898, I first observed a minute nematode parasite wrig- 
gling about in the central lacunar space of a live G. stagnalis. Another 
similarly parasitized leech was found upon further search, and a third 
was found in the following March, the ovary of the host containing at 
that time full-grown eggs. ‘The parasite in the last-mentioned case lay 
close to the contractile dorsal blood-vessel, a very common position for 
it, as subsequent observations showed. In the spring of 1899 several 
parasitized individuals were collected and studied; and others were 
observed in the fall of 1899. 
The length of the parasite is about the same in the case of all 
individuals examined ; namely, 1.43 mm. In form, the worm is slender 
and thread-like, being widest near the middle of its body, where it 
measures 0.027 mm. in breadth. From there it tapers almost imper- 
ceptibly toward either end. The posterior end of the body is sharply 
pointed ; the anterior end blunt, its centre being occupied by the very 
minute, conical mouth. 
Examination of a large number of individuals of G. stagnalis in the 
spring of 1899 showed that between five and ten per cent of the indi- 
viduals taken from a particular pond, in which the species abounds, 
contained the nematode parasite. Usually only a single parasite has 
been observed in the body of a host, but in one case there were three. 
The nematode is generally found either coiled up (but not encysted) or 
wriggling about in the central lacuna (body cavity), in the middle or 
toward the posterior end of the body. The presence of the parasite 
does not seem seriously to inconvenience its host, for the parasitized 
individuals are as large and well developed as those free from parasites, 
and contain sexual products in equal abundance. 
Parasitized individuals were-kept in aquaria for several weeks without 
the occurrence of any noticeable change in the condition of the parasites. 
This fact and the manifest immaturity of all the parasites examined 
makes me believe that the leech is an intermediate host and that the 
